Conventional rotor-craft sometimes use multiple propellers (i.e., rotors) that are sometimes symmetrically located relative to a center of mass of the aircraft. Each propeller is coupled to a different drive shaft and powered by a different single motor using an electronic speed control (ESC), which controls the rotational speed of the motor, and thus the rotational speed of the propeller. Some rotor-crafts have four, six, eight, or more propellers.
When many propellers are used for propulsion of a rotor-craft, the rotor-craft may have some level or redundancy in case of failure of a motor or damage to a propeller. For example, an octocopter may continue controlled flight in the event of a failure of one of the motors that drive a particular propeller or damage to the particular propeller because the other seven propellers can typically maintain flight of the octocopter even when one propeller is no longer completely functional. However, rotor-craft having fewer propellers may not be able to maintain controlled flight in the event of a failure of a motor or damage to a propeller.
Propeller blades, like wings, create vortices during rotation of the propeller blades. The vortices create drag, which slows the propeller and causes inefficiency. In addition, propeller blades often create undesirable noise during operation at high rotational speeds at least partly due to airflow about the tips of blades of the propeller. When heavily loaded, propeller blades may deflect, which may reduce some efficiency of the propellers, and may even result in failure of the propeller if the deflection compromises the structural integrity of the propeller.